PC shut downs when i play CoD MW2

Hi guys.My PC shut downs by itself, can be strait away, 15min, 30min,..... after i start game CoD MW2. It does not crash to desctop, but shuts my PC. I managed to play it before after like 10s shut downs in the row.

Graphic card 8500GT 1GB It start happening when i instaled new graphic card GeForce 8500 GT 1GB. When i remove card every thing is back to normal. But i need this graphic card for games.Withot it can not play as everything is so slow.Please help.I do not know what to do, tryed everything, nothing works. Feel like throving my PC out of the window. Help if you can, i really like CoD MW2 multiplayer, but can not play it.If you got any ideas please let me know.Thank you.

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